Laser irradiation of semiconductor material surfaces is well known for applications such as thermal annealing of amorphous silicon to obtain re-crystallization, and dopant activation. This technique offers significant advantages over a conventional heating process by enabling a very fast heat treatment and shallow depth of the heated region.
A general problem of conventional laser irradiation processes for semiconductor applications is that due to the high energy density required for the thermal process and the low output energy of traditionally available laser sources, the laser spot size is much smaller than the size of a die, also called a chip or device. As a consequence, the laser spot has to step over or scan the die to cover the full die, resulting in several disadvantages
As described by Current and Borland (Technologies New Metrology for Annealing of USJ and Thin Films, 16th IEEE International Conference on Advanced Thermal Processing of Semiconductors—RTP2008) and as illustrated in FIG. 1 and FIG. 2, a first disadvantage is that, if the laser spot (b) scans or steps over the die (a), successive laser spots will overlap at some portions (c) of the die, leading to non-uniformities in dopant activation rate or depth and in surface quality.
Another disadvantage is that, in case multiple laser pulses on the same surface area are needed, the laser spots scans or steps over the surface with very high overlap in order to average multiple laser pulses on each point of the processed surface area, resulting in limited production rate and periodic non-uniformities, so called moiré-patterns.
Another general problem is that dies for different types of applications usually have different sizes, and further that in some applications only parts of the die have to be irradiated. It is well known by the skilled person that, in order to be able to process different die sizes or parts of dies with limited overlap, the beam spot is shaped by a variety of masks with different sizes. Since consequently the mask has to be changed and tuned each time another size is needed, manufacturing flexibility is severely limited and downtimes may be significant.
In an attempt to overcome the above drawbacks, WO 01/61407 (Hawryluk et al.) describes a laser irradiation apparatus using a variable aperture stop for defining the size of the exposure field.
A clear drawback however is that, according to Hawryluk et al., the laser light source needed to obtain satisfying uniformity needs to be a solid-state laser with more than 1000 spatial modes, which is not a currently commercially available laser source.
Another example where the beam spot is sized is US 2006/0176920, wherein Park et al. describe a laser irradiation apparatus comprising intensity pattern regulating units having a through, a semi-through and a blocking region for variably regulating the intensity of a strip-shaped laser beam by regulating its length.
Considering the drawbacks of the above laser irradiation processes, there is a clear need for the laser irradiation method and apparatus according to the present invention, which as a first object may provide the ability to process semiconductor material layers obtaining acceptable uniformity within die and within wafer, while keeping acceptable production rate, and manufacturing flexibility.
As second object the present invention may provide a reduction of overlapping effects and attenuation regions.
As another object the present invention may provide the ability to generate a beam with flexible image shape on the material layer surface.
As another object the present invention may provide the ability to irradiate at lower temperatures and maximize the conversion of laser energy into heat.
The present invention meets the above objects by variably matching the laser beam spot size to the selected region size.